A craniotomy is an operation involving a cut that opens the cranium, During this surgical procedure, a section of the skull, called a bone flap, is removed to access the brain underneath. The bone flap is usually replaced after the procedure with tiny plastes and screws. If it's not is called craniectomy.
It may be performed during surgery for various neurological diseases, injuries or conditions such as brain tumors, hematomas, aneurysms or AVMs, and skull fractures. Other reasons for a craniotomy may include foreign objects(bullets) swelling of the brain (cerebral oedema) or infection.
Small dime-sized craniotomies are called burr holes or keyhole craniotomies and are used for minimally invasive procedures such as to insert an intracranial pressure monitor, insert and endoscope to remove small tumors and clip aneurysms, insert a shunt into the ventricles to drain cerebrospinal fluid, insert a deep brain stimulator to treat Parkinson disease, drain a blood clot (stereotactic hematoma aspiration) or needle biopsies.